The third generation partnership project (3GPP) has standardized Long Term Evolution (LTE) that is a wireless communication standard for the 3.9 generation cellular technology. As illustrated in FIG. 1, for LTE, standard specifications have been established to exchange load information between base stations (eNB) 1 and 2 using an inter-base station interface (X2 interface). Information elements of the load information are periodically exchanged between the base stations. The information elements are (1) downlink (DL)/uplink (UL) guaranteed bit rate (GBR) physical resource block (PRB) usage (i.e., wireless resource usage of a guaranteed bit rate call such as an audio call; (2) DL/UL non-GBR PRB usage (i.e., wireless resource usage of a best-effort call such as web browsing); (3) DL/UL total PRB usage (i.e., wireless resource usage of all calls); (4) a DL/UL hardware load indicator; and (5) a DL/UL S1 transport network layer (S1TNL) load indicator (i.e., a wired load indicator).
In handover (HO) load balancing optimization that is one of use cases of a self-optimization network (SON), a handover parameter is changed to prompt a mobile station (user equipment (UE)) to perform handover from a high-load base station to a low-load base station in order to balance loads.
The handover parameter is an offset value that is added to the quality of a signal wirelessly received from a source base station (serving base station) for the quality of a signal wirelessly received from a destination base station (target base station). The destination base station is a handover destination, while the source base station is a handover source. As indicated by the following formula, as the offset value becomes smaller, the mobile station more readily performs handover to the destination base station.
The quality of the signal wirelessly received from the destination base station>the quality of the signal wirelessly received from the source base station+the offset value
An offset value can be set in a certain source base station for each of a plurality of destination base stations.
The source base station transmits the offset value to the mobile station. The mobile station measures the qualities of signals wirelessly received from the destination base stations. When the measurement result satisfies the aforementioned formula, the mobile station transmits, to the source base station, a report that indicates the measurement result. Then, the source base station that receives the report specifies a destination base station satisfying the aforementioned formula as a candidate for a handover destination base station. Then, the source base station instructs, on the basis of the result of call admission control (described later), the mobile station to perform handover to the destination base station.
For example, in FIG. 1, the high-load base station 2 changes a handover parameter and prompts the mobile station 3 to perform handover to the low-load base station 1, and the mobile station 3 performs handover from the base station 2 to the base station 1.
FIG. 2 is a call flow diagram illustrating conventional call admission control. In FIG. 2, a source base station 1A that is a handover source of a certain mobile station selects, as candidates for a handover destination base station, a plurality of base stations 2A and 2B (satisfying the aforementioned formula, for example) from which a mobile station 3A can wirelessly receive signals with certain or higher qualities (in steps S1 to S3). The source base station 1A transmits handover requests (HO requests) to the destination base stations (2A and 2B) (in step S4).
The destination base stations 2A and 2B confirm loads of the destination base stations 2A and 2B, respectively (in step S5). When the base station 2A can admit the mobile station 3A, the base station 2A transmits, to the source base station 1A, a response (HO Request Ack) that indicates that the admission is permitted (in step S6). When the base station 2B cannot admit the mobile station 3A, the base station 2B transmits, to the source base station 1A, a response (HO Request Ack) that indicates that the admission is rejected (in step S7).
The source base station 1A transmits a handover command (HO command) to the mobile station 3A and instructs the mobile station 3A to perform handover to the base station 2A that permits the admission and is specified as a handover destination base station (in step S8). The series of admission control is generally called call admission control (CAC). The mobile station 3A performs handover to the specified destination base station 2A (in steps S9 and S10).
The following technique (refer to, for example, Japanese Unexamined Patent Application Publication No. 2001-169323) is known. The technique is to learn the qualities of signals that are data that is obtained between a mobile station and a plurality of base stations in the past and chronologically arranged, learn past handover destinations, and select a target base station on the basis of similarities between the qualities of signals that are data currently obtained and chronologically arranged and the qualities of the signals that are data obtained in the past and chronologically arranged.
In addition, the following technique (refer to, for example, Japanese Unexamined Patent Application Publication No. 7-236173) is known. The technique is to classify all traffic channels into a first channel group of traffic channels having the same priority and a second channel group of traffic channels each having priorities that are specific to base stations, change the priorities on the basis of measured levels of interference waves of the traffic channels of the second channel group, measure desired-to-interference wave ratios of traffic channels in order from a traffic channel having the highest priority at the time of a request for an assignment of a traffic channel, and assign the traffic channel when the desired-to-interference wave ratio of the traffic channel is equal to or higher than a predetermined threshold.
In addition, the following technique (refer to, for example, Japanese Unexamined Patent Application Publication No. 2008-16970) is known. The technique is to generate handover history information (that includes information of a cell used for communication every time a mobile terminal performs handover; and successful and failed results of handover performed when a preprocess is not performed), calculate a failure rate of handover to a handover destination cell, and perform the preprocess for the handover destination cell when the failure rate is high.